For all its triple-crosses, coups, scams and swindles, Once Upon A Time in Mexico boils down to one fact: El Mariachi is going to have to, as he simplistically puts it, kill everyone.

Filmmaker Robert Rodriguez constantly threatens to mire this, his third El Mariachi movie, in an inescapable muck of exposition. But he ultimately remembers killing — and, more dramatically and breathlessly, avenging — is what his characters do best.

A crackling energy pulses through Mexico’s final 45 minutes, one that elevates it to the joyful pulp status of its predecessors. And Johnny Depp, method-acting genius that he is, gleefully feeds off it, not so much stealing the movie as taking a very active role in its third-act triumph.

Antonio Banderas returns as El Mariachi (“El,” for short), last seen riding off into the sunset with the very beautiful Carolina (Salma Hayek). As we learn here in flashbacks, their honeymoon was a short one and El becomes a gun-for-hire for rogue, disguise-happy CIA agent Sands (Depp).

There is a coup d’etat planned by drug kingpin Barillo, played by Willem Dafoe in a performance that shoots humorous daggers at Charlton Heston’s ridiculous turn as a Mexican in Touch of Evil. Dafoe is funnier and much more subtle than when Depp, cloaked as a priest, does Brando.

Sparing you the convolution, it boils down to Sands wanting to make off with a bunch of blood money and El Mariachi looking for personal revenge against a foe that took quite a bit from him.

Rodriguez gets a glorious visual early on, a wide establishing view of a village square that has a dusty, foreboding quality straight from The Wild Bunch or, more recently, The Way of the Gun. And at least most of the rest of the movie has bright and vibrant colors.

The film was shot on high-definition digital video, which helps Rodriguez keep his standard low bottom-line but can look atrocious. At times, a combination of red-and-blue at the edge of the screen looks like Rodriguez’s Spy Kids 3-D without the glasses. Also, a hospital shootout is as clear as al-Jazeera footage with sand engrained into the tape and explosions look oddly pixilated.

Plus, there are just so many characters that it’s hard to keep track of them, let alone give a hoot. There is not one good reason for a subplot of brutal length involving Mickey Rourke and Rubén Blades, other than needing them to show up in the climactic gunfight. And Enrique Iglesias as a mariachi-cum-gunslinger is precisely as miscast in the role as you would expect.

But when Rodriguez uncorks, he does so masterfully, in a handful of action sequences that pop, including Banderas and Hayek playing human Plinko down the side of a building and a dirt-bike chase. In the film’s many gunfights, people go down with what looks like a flare exploding from their chests, and El Mariachi’s standoff with his longtime nemesis features a kneecapping that’s one of the best drop-your-jaw, action-movie money shots of all time.

And then there’s Depp, who certainly doesn’t look like he’s working hard, but darn it if he’s not as great here as he was in Pirates of the Caribbean. As Sands waxes on lovingly about a Mexican pork dish, Depp turns him into a whack-job version of Agent Dale Cooper from Twin Peaks, not above murdering cooks, bullfighters or anyone else who crosses him. Sands is a scumbag, but Rodriguez realizes the audience likes him and, in a matter of speaking, turns him into an anti-hero.

Without spoiling anything, suffice it to say that when Once Upon A Time in Mexico is over, the first thing you’ll think is how you’d love to see Depp and that kid in another movie. Rodriguez may do nothing but all pulp, all the time, but he delivers it with unquestionable zest and skill here.